In recent years, the speed with which electronic calculators and information processing systems can process ever greater amounts of information has increased sharply. Together with the digitalization of audio and video information, this gave rise to the rapid dissemination of low-cost, high-volume auxiliary storage devices and recording media therefor, especially optical disks, which can be accessed with high access speeds.
The basic configuration of conventional optical disks is as follows: A dielectric layer is formed on top of a disk substrate, and a recording layer is formed on top of the dielectric layer. On top of the recording layer, an intermediate dielectric layer and a reflecting layer are formed in that order. An overcoat layer is formed on top of the reflecting layer.
The following is an explanation of how an optical disk with the above configuration is operated.
In the case of an optical disk having, in its recording layer, a magneto-optical layer with perpendicular magnetic anisotropy, the recording and erasing of information is performed by locally (a) heating the recording layer with a laser beam to a temperature with small coercive force above the compensation temperature or to a temperature near or above the Curie temperature to decrease the coercive force of the recording layer in the irradiated portion, and (b) magnetizing the recording layer in the direction of an external magnetic field. (This is also called “thermomagnetic recording” of information.). Moreover, for the reproduction of the recording signal, a laser beam with less intensity than the laser beam for recording or erasing irradiates the recording layer. The recording state of the recording layer, that is, the rotation of the polarization plane of the light that is reflected or transmitted in accordance with the orientation of the magnetic field (this rotation occurs mainly due to two magneto-optical effects—the Kerr effect and the Faraday effect), is detected by a photodetector through the change in the intensity of the irradiated light. In order to decrease the interference between opposite magnetizations and allow high-density recordings, a magnetic material with perpendicular magnetic anisotropy is used for the recording layer of the optical disk.
Moreover, when the data is reproduced, the reproduction signal level during data reproduction can be raised to detect the reproduction signal by using a layered structure for the recording layer: Several magnetic thin films comprising an exchange coupling multilayer or a magneto-static coupling multilayer.
For the recording layer, a material is used that can record information by locally raising the temperature or inducing a chemical reaction due to absorption of the irradiated laser light. The local variations in the recording layer can be detected by irradiating laser light of a different intensity or wavelength than that used for the recording and detecting the reproduction signal using the reflected or the transmitted light.
Regarding such optical disks, there is a need for a way to protect the data on the disk with write-once information (identification data) that allows for copyright protection, for example copy protection and protection against unauthorized use of software.
With the above configuration, it is possible to record disk information in TOC (or control data) areas, but when disk data is recorded with pre-pits, the disk information has to be administered stamper by stamper and cannot be administered user by user.
Moreover, when information is recorded using a magnetic film or a film of a phase-reversible material, administrative information easily can be changed, which means that it easily can be rewritten (manipulated), so that the contents on the optical disk cannot be copyright protected.